Biomass pyrolysis in itself is not a new technology and the literature abounds with examples of various types of pyrolysis units. It is generally accepted that biomass pyrolysis can be carried out in either a fast mode or a slow mode. The fast mode maximizes liquid yield, while the slow mode maximizes solid (charcoal) yield. Lede (2013) published an extensive review and description of fast pyrolysis systems. Freel (2017) compiled an extensive list of references as part of U.S. Pat. No. 9,631,145.The extensive reference list is testimony to the volume of literature available on pyrolysis. The following is a list of fast pyrolysis processes including but not restricted to circulating bed (Freel), fluid bed (Piskorz et al,) twin auger (Brown, Henrich et al) or single auger (Hornung, Fransham) Virtually all fast pyrolysis systems involve mixing the biomass with a heat carrier. Silica sand is the most often used, although steel shot (Fransham), steel balls (Hornung, Poulleau et al) or ceramic shot is used in most auger pyrolysis. When the heat carrier is circulated with recycled gas, sand is the heat carrier of choice. The general object of fast pyrolysis is to drive off volatiles from biomass material and condense them in a matter of seconds. Slow pyrolysis processes include augers (Flottvik, Poulleau et al), multiple hearth and batch systems used in the coking industry. Slow pyrolysis systems do not generally use a heat carrier and is generally carried out in a matter of minutes. People skilled in the art will recognize that other possible processes also exist for conducting pyrolysis.
Circulating bed type fast pyrolysis systems (Freel) also known as transport bed processes involve moving sand vertically in a tube at velocities in the order of 20 m/sec. The motivating gas is oxygen free recycled gas obtained after all of the condensable volatiles have been stripped from the non-condensing gas. Biomass enters the vertical tube at a point above the base of the tube. The biomass mixes with the sand in the tube and the volatile matter in the biomass is converted to a hot gas. There are however, fundamental problems with transport beds. The first is the use of sand as a heat carrier. Sand has a low thermal conductivity and the sand temperature has to be high enough to transfer sufficient energy to raise the biomass temperature to approximately 515° C. However, the short contact time and the low thermal conductivity of sand mean that only a fraction of the energy contained in the sand grain can be transferred to the biomass. Also, the large amount of recycled gas that is required for transport is a parasitic load on the system. The gas has to be heated and cooled for each cycle. Condensers have to be large enough to handle the heat load in the gas stream. The excessive electrical energy required to transport sand several meters vertically in the air at velocities of about 20 m/sec. greatly reduces the efficiency of the process.
Furthermore, the sand and biomass are in a dispersed low density mixture of sand, biomass and motivating gas. Heat transfer from the sand to the biomass is predicated on the random contact between the sand grains and the biomass particles.
Fluid bed pyrolysis processes have similar short comings to circulating bed reactors. Fluid beds have to be shallow to ensure short vapour residence times required to limit secondary chemical reactions. Preventing carryover of the sand into the char recovery circuit requires balancing of sand size and airflow. A further limitation is the transferring of heat into the bed. The only ability to do pyrolysis work is dictated by the mass of gas multiplied by the temperature differential between the incoming and exiting recycle gas multiplied by the specific heat of the recycle gas. Large blowers are required to move the recycled fluidizing gas. The key technical challenge is to scale up the reactor to meet the demands of short residence time while maintaining the sand in the reactor. The sand in the fluid bed is in a low density medium and contact between the sand particle and the incoming biomass relies on rapid mixing before a large bubble of gas rises to the bed surface and expands outwardly. There is therefore a considerable technical challenge to feeding biomass into large reactors and separating reacted biomass (charcoal) from sand. Attempts have been made to scale up fluid bed reactors for pyrolysis. No biofuel pyrolysis plant is currently operating at a commercial scale using this technology.
Auger pyrolysis offers a solution to the deficiencies in the fluid bed and circulating bed processes. Steel shot has a higher thermal conductivity than sand and hence more energy can be transferred at a lower operating temperature from the shot to the biomass in an equivalent period of time as compared to sand reactors. Fransham (2001) developed an auger system whereby the shot was recirculated via a bucket elevator. The charcoal and non-condensing gases were burned to provide process heat. The system relied on pressure from the expanding raw pyrolysis gas and the volume reduction in the gas at the first stage condensing unit to move the gas from the reactor to the condensers with a residence time similar to that of the other pyrolysis systems. The advantage of the auger system is the biomass is in close contact with the heat carrier and hence high heating rates are achieved. The use of steel shot over sand significantly reduces auger wear and heat carrier attrition when compared to sand filled reactors.
Hornung used hollow steel balls to rapidly pyrolyze biomass and separated the charcoal from the balls in a trommel screen. The balls were heated and circulated in a system of screw conveyors. This system has been used for a variety of applications and numerous papers have been written and patents filed on results from this process. Poulleau et al developed a complex system of augers to move steel balls in a pyrolysis apparatus. The system requires controlled feeding of the steel balls into an auger reactor along with controlled biomass feeding. The biochar is separated through a screen. The steel balls are heated separately in a furnace. The apparatus is designed to produce a maximum amount of high calorific gas and by common definition is a gasifier and not a pyrolysis process to maximize biooil yield. Poulleau's apparatus uses an auger configuration that is more complicated than that of the present invention. The present invention, moreover, is a two stage pyrolysis system whereby pyrolysis occurs rapidly in the first auger and more slowly in the second auger. The advantage of the present invention is the maximization of liquid yield and minimization of biochar and non-condensable gas yield.